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Nasdaq's Apple move to drag on stocks

NEW YORK (CNNMoney) -- U.S. stocks are gearing up for losses at the opening bell Tuesday, as investors mull over the rebalancing of the Nasdaq-100 index to diminish Apple's influence, a rate hike in China and another downgrade of Portugal's debt.

"Those investors who've got a portfolio tracking that index will need to sell Apple shares to rebalance their portfolio," said James Cordwell, digital equity analyst with Atlantic Equities in London. "Investors would have to shift from Apple to Oracle and Microsoft."

The Nasdaq-100 (NDX) includes the 100 largest nonfinancial companies listed on the Nasdaq exchange -- not to be confused with the Nasdaq Composite, which includes all of the roughly 3,000 companies listed on the exchange.

Meanwhile, Europe's debt crisis is back in focus, after Moody's Investors Service downgraded Portugal's debt to a lower investment grade rating.

Among various reasons for the change, the ratings agency cited Portugal's "uncertain political outlook," following Jose Socrates' resignation as the country's prime minister two weeks ago.

"The announcement may cause jitters about the impact tightening will have on Chinese growth. However these should not be overplayed," Mark Williams, senior China economist with Capital Economics, said in a research note. "The latest increases... are in line with the gradual policy tightening that has been underway over the last few months and will not do much to slow the economy."

World markets: European stocks fell in midday trading. Both Britain's FTSE 100 and the DAX in Germany fell 0.3%, while France's CAC 40 lost 0.6%.

Japan's Nikkei ended 1% lower. Markets in Shanghai and Hong Kong were both closed for a holiday.

Economy: At 10 a.m. ET, the Institute for Supply Management will release its monthly gauge on the service-sector index for March. Economists are looking for the index to slip slightly to 59.5, compared with last month's reading of 59.7, a level that would still indicate expansion in the sector.

Later in the day, the Federal Reserve will release minutes from its March 15 policy meeting.

Companies: KB Home (KBH) shares fell nearly 6% after the homebuilder announced a quarterly loss of $114.5 million, or $1.49 a share. That's far deeper than the loss analysts were expecting.

Shares of rival homebuilder Lennar Corp. (LEN) fell 0.5% in early trading after the announcement.

Shares of National Semiconductor (NSM) surged 71% in premarket trading. Late Monday, the company agreed to be acquired by Texas Instruments (TXN, Fortune 500) for $6.5 billion in cash. Shares of Texas Instruments fell 2.4%.

Shares of Expedia (EXPE) rose almost 5% in early trading, after American Airlines announced the two companies will resume doing business together after reaching an agreement on airfare sales.